1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor and, particularly, to a solid electrolytic capacitor which is constituted by forming at least a solid high molecular polymer electrolyte layer and a conductive layer on a foil-like valve metal substrate formed with an insulating oxide film on the surface thereof and can reduce the ESL and the ESR and increase electrostatic capacitance at a small size, and a method for manufacturing such a solid electrolytic capacitor.
2. Description of the Related Art
An electrolytic capacitor is conventionally formed by employing a so-called valve metal capable of forming an insulating oxide film such as aluminum, titanium, brass, nickel, tantalum or the like as an anode, anodizing the surface of the valve metal to form an insulating oxide film thereon, forming an electrolyte layer substantially serving as a cathode, and forming a conductive layer of graphite, silver or the like as a cathode.
For example, an aluminum electrolytic capacitor is formed by employing as an anode a porous aluminum foil whose specific surface area is increased by etching, and providing a separation paper impregnated with an electrolytic solution between an aluminum oxide layer formed on the surface of the anode and a cathode foil.
In general, although an electrolytic capacitor using an electrolytic solution for an electrolyte layer between an insulating oxide film and a cathode has the disadvantage that its lifetime is determined by liquid leakage, evaporation of the electrolytic solution and the like, a solid electrolytic capacitor using a solid electrolyte containing metal oxide or organic compounds does not have such a disadvantage and is preferable.
Manganese dioxide is a typical metal oxide usable for the solid electrolyte of a solid electrolytic capacitor. On the other hand, as an organic compound usable for the solid electrolyte of a solid electrolytic capacitor, 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex salt disclosed in Japanese Patent Application Laid Open No. 52-79255 and Japanese Patent Application Laid Open No. 58-191414 can be cited.
Recently, as the frequency of power circuits of electronic devices has become higher, corresponding performance is required of capacitors used therewith. However, a solid electrolytic capacitor using a solid electrolyte layer containing manganese dioxide or TCNQ complex salt has the following disadvantages.
Although a solid electrolyte layer containing manganese dioxide is generally formed by repeating thermal decomposition of manganese nitrate, an insulating oxide film is damaged or degraded by heat applied during thermal decomposition or oxidative effect of NOx gas generated during thermal decomposition. Therefore, in the case where a solid electrolyte layer is formed using manganese dioxide, leakage current becomes high, for example, and various characteristics of the capacitor finally obtained tend to be lowered. Further, in the case where a solid electrolyte layer is formed using manganese dioxide, the solid electrolytic capacitor has the disadvantage that impedance thereof becomes higher in the high frequency region.
On the other hand, a solid electrolytic capacitor using TCNQ complex salt does not adequately satisfy the requirement for low impedance of current solid electrolytic capacitors, since the electric conductivity of TCNQ complex salt is about 1 S/cm or lower. It has been further pointed out that the reliability of a solid electrolytic capacitor using TCNQ complex salt as a solid electrolyte is not sufficient for the reason that adhesive strength between TCNQ complex salt and an insulating oxide film is low and the thermal stability of TCNQ complex salt during soldering and with lapse of time is low, and the like. In addition, TCNQ complex salt is expensive and, therefore, the cost of a solid electrolytic capacitor using TCNQ complex salt as a solid electrolyte is high.
For solving these problems occurring when manganese dioxide or TCNQ complex salt is used as a solid electrolyte and obtaining a solid electrolytic capacitor having better characteristics, it has been proposed to use as a solid electrolyte a high molecular compound having high electric conductivity whose manufacturing cost is relatively low, whose adhesive strength to an insulating oxide film is relatively good and whose thermal stability is excellent.
For example, Japanese Patent No. 2,725,553 discloses a solid electrolytic capacitor in which polyaniline formed on an insulating oxide film on the surface of an anode by the chemical oxidation polymerization process.
Further, Japanese Patent Publication No. 8-31400 proposes a solid electrolytic capacitor in which a thin film of metal or manganese dioxide is formed on an insulating oxide film and a conductive polymer film of polypyrrole, polythiophene, polyaniline, polyfuran or the like is formed on the thin film of metal or manganese dioxide by the electrolysis polymerization process, for the reason that it is difficult to form a conductive polymer film having high strength on an insulating oxide film on the surface of an anode only by the chemical oxidation polymerization process and that it is impossible or extremely difficult to directly form an electrolysis polymerization film on an insulating oxide film on the surface of an anode by the electrolysis polymerization process because the insulating oxide film on the surface of an anode is a non-conductor.
Furthermore, Japanese Patent Publication No. 4-74853 proposes a solid electrolytic capacitor in which a conductive polymer film of polypyrrole, polythiophene, polyaniline, polyfuran or the like is formed on an insulating oxide film by the chemical oxidation polymerization process.
Further, in order to reduce impedance, it is necessary to lower the equivalent series inductance (ESL) and equivalent series resistance (ESR) of capacitors used in the electronic devices and it is particularly necessary to sufficiently lower the ESL in the electronic devices including the power circuits of low frequency. As methods for decreasing the ESL, there are generally known a first method of shortening the length of the electric path as possible, a second method of canceling the magnetic field generated by one electric path by the magnetic field generated by another electric path, and a third method of dividing an electric path into n pieces to decrease the effective ESL to 1/n. For example, the first method and the third method are employed in the invention taught by Japanese Patent Application Laid Open No. 2000-311832, the second method and the third method are employed in the invention taught by Japanese Patent Application Laid Open No. 06-267802 and the third method is employed in the inventions taught by Japanese Patent Application Laid Open No. 06-267801 and Japanese Patent Application Laid Open No. 11-288846.
At the same time, the frequency of the power circuits of electronic devices has recently become higher and this has made it necessary to lower the equivalent series inductance (ESL) and equivalent series resistance (ESR) of capacitors used in the electronic devices. Even if the initial characteristic ESL value or the like can be markedly improved, such requirement cannot be practically satisfied in the case where the characteristic value is liable to vary in a reliability test such as a high temperature application test or the like. Therefore, it is required to develop an electrolytic capacitor in which initial characteristic ESL and ESR values are very low and do not vary substantially.